feat: sticky table of contents with scrollspy for canvas dashboards

[rohithreddykota]

Adds a persistent, minimap-style table of contents to canvas dashboards (view surfaces only; not the iframe embed).

• Derives entries from the headings (h1–h3) rendered inside markdown components of the active tab — no new section primitive; stays in sync as content/tabs change via a debounced MutationObserver.
• Scrollspy via a single IntersectionObserver (no scroll-event math); exactly one active section, with the highlight pinned during click-to-scroll so it doesn't flicker through intermediate sections.
• Collapsed rail of tick bars by default (dots on narrow viewports so they don't overlap content); hover/focus reveals the full list as an overlay flyout that never reflows the dashboard.
• Click smooth-scrolls (respecting prefers-reduced-motion) and updates the URL hash; deep-linking on mount is supported. Accessible: <nav aria-label>, real links, aria-current="location", focus rings, full heading as the accessible name.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code

[AdityaHegde]

A couple of UXQA items,

1. When there are 2 markdown components side-by-side,
   1. Clicking on the right one in sidebar will not highlight it, only the left one gets highlighted.
   2. Scrolling just down will quickly highlight the right one.
      Not sure if there is a legitimate use case for this, so perhaps we can keep the existing behaviour.
2. When the last markdown component is towards the bottom it never gets highlighted. But clicking it does take the user to the bottom so probably not a bad experience?

What do you think @nishantmonu51 ?

[AdityaHegde]

Lets use toc-panel. The naming convention of double underscore is not standard for css. Same applies to double dash classes like toc__item--active.

[AdityaHegde]

We can move this and watchScrollEnd to a separate class that has logic from loc and scrollspy. Also we should move to svelte5 way for using cross file state. Check web-common/src/lib/store-utils/types.svelte.ts for reference.